Novel phenoxazine-benzonitrile and phenothiazine-benzonitrile donor-acceptor molecules with thermally activated delayed fluorescence (TADF)

Article abstract of DOI:10.1016/j.dyepig.2019.108114

Four novel TADF emitters, containing phenothiazine and phenoxazine as electron-donors and benzonitrile derivatives as electron-acceptors were synthesized and fully characterized. Their photophysical (absorption and emission spectra, molar extinction coeff

Full text of DOI:10.1016/j.dyepig.2019.108114

Journal Pre-proof
Novel phenoxazine-benzonitrile and phenothiazine-benzonitrile donor-acceptor
molecules with thermally activated delayed fluorescence (TADF)
Faiza Baraket, Bruno Pedras, Érica Torres, Maria João Brites, Mohamed Dammak,
Mário N. Berberan-Santos
PII:
S0143-7208(19)32563-X
DOI:
https://doi.org/10.1016/j.dyepig.2019.108114
DYPI 108114
Reference:
To appear in: Dyes and Pigments
Received Date: 31 October 2019
Revised Date: 4 December 2019
Accepted Date: 6 December 2019
Please cite this article as: Baraket F, Pedras B, Torres É, Brites MariaJoã, Dammak M, Berberan-
Santos MáN, Novel phenoxazine-benzonitrile and phenothiazine-benzonitrile donor-acceptor molecules
with thermally activated delayed fluorescence (TADF), Dyes and Pigments (2020), doi: https://
doi.org/10.1016/j.dyepig.2019.108114.
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Novel phenoxazine-benzonitrile and phenothiazine-benzonitrile
donor-acceptor molecules with thermally activated delayed
fluorescence (TADF)
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Faiza Baraketa,b, Bruno Pedrasa,*, Érica Torresa,c,‡, Maria João Britesc, Mohamed
Dammakb, Mário N. Berberan-Santosa
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aCQFM-IN and iBB-Institute for Bioengineering and Biosciences, Instituto Superior
Técnico, Universidade de Lisboa, 1049-001 Lisboa, Portugal
bLaboratory of Inorganic Chemistry LR17ES07, Faculty of Sciences of Sfax, University
of Sfax, 3000 Sfax, Tunisia
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cLaboratório Nacional de Energia e Geologia, I. P., UER-Unidade de Energias
Renováveis e Integração de sistemas de Energia, Edifício G, 1º andar, Estrada do
Paço do Lumiar, 22, 1649-038 Lisboa, Portugal
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* Corresponding author. E-mail address: bruno.pedras@tecnico.ulisboa.pt
‡Present address: Centro de Química Estrutural and Institute of Nanosciences and
Nanotechnology, Instituto Superior Técnico, Universidade de Lisboa, 1049-001 Lisboa,
Portugal.
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Abstract
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Four novel TADF emitters, containing phenothiazine and phenoxazine as electron-
donors and benzonitrile derivatives as electron-acceptors were synthesized and fully
characterized. Their photophysical (absorption and emission spectra, molar extinction
coefficients, fluorescence quantum yields and lifetimes) and electrochemical properties (HOMO
and LUMO energy levels) were measured, and drop-cast solid films of the four compounds
were obtained to perform TADF studies. The obtained values for ∆EST indicate that these
compounds are candidates for OLED applications.
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Keywords: TADF, OLED, Fluorescence, Phenoxazine, Phenothiazine.
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Introduction
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Luminescent materials for organic light-emitting diodes (OLEDs) have been an
important subject of both academic and industrial research in recent years. Organic
fluorescence emitters with Thermally Activated Delayed Fluorescence (TADF) have attracted
much attention in that these materials use thermally activated up-conversion of triplet (T1) into
singlet (S1) state, giving stable fluorescence with very high fluorescence yields [1-3]. These
TADF materials, used in third generation Organic Light Emitting Diodes (OLEDs) by charge-
transfer (CT) excited state [4,5], require a sufficiently small energy gap between the triplet and
singlet (∆EST) to enable up-conversion of the triplet excitons to singlet excitons and realize
100% internal quantum efficiency [6,7] of the exciton formation generated by electric excitation
at S1. Thus, enormous efforts have been endeavored to minimize ∆EST and simultaneously
enhancing the reverse intersystem crossing (RISC) by molecular structure design, leading to
the maximization of TADF. Nowadays, particularly designed organic materials with donor-
acceptor structures are used to minimize the HOMO-LUMO overlap.
Versatile molecular systems with TADF have been developed, including acridine [8],
carbazole [9], phenoxazine [10-12], phenothiazine and diphenylamine as donor groups and
triazine [13], cyanobenzene, benzophenone and diphenyl sulfone derivatives [14] as acceptor
groups. High external quantum efficiencies (EQE) up to 20% have been reported for OLEDs
using TADF materials as emitters, which come close to the best performance of OLEDs based
on organic phosphorescent materials reported.
Phenoxazine and phenothiazine are potentially good electron donors and hence have
been used extensively as fluorescent emitters in first generation OLEDs [15-18], dye sensitized
solar cells [19] and laser dyes [20]. However, because these materials are conventional
fluorescent emitters rather than TADF ones, the EQEs obtained for their devices are less than
those of phosphorescent OLEDs. In these fluorescent compounds, the large overlap between
HOMO and LUMO is a major factor preventing the existence of TADF. In general, the Highest
Occupied Molecular Orbital (HOMO) is localized in the electron donor moiety and the Lowest
Unoccupied Molecular Orbital (LUMO) in the electron acceptor moiety.
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Based on the aforementioned, in this work we report the design, synthesis and
characterization of four novel donor-acceptor (DA) TADF emitters, containing phenothiazine and
phenoxazine as electron-donors and 4-fluoro-2-(fluoromethyl)-benzonitrile and 4-
bromobenzonitrile as electron-acceptors. In particular, we focus upon the methods that are
currently in use to characterize the TADF mechanism, and determine the photophysical and
electrochemical parameters that are useful to correlate the observation of TADF with molecular
structure. All four studied DA compounds display TADF at temperatures well below room
temperature.
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2. Experimental section
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2.1. Materials
Chemicals were purchased from commercial sources and used without further purification.
Solvents were dried and distilled immediately prior to use by standard procedures [11]. Vacuum
filtration purification was performed using silica gel (230-400 mesh) as the stationary phase in a
filter funnel with porous plate. Tetrabutylammonium hexafluorophosphate (TBAPF6-98%) used
for cyclic voltammetry (CV) was purified by recrystallization from ethanol and dried under
vacuum prior to use. The polymeric films were prepared from cyclic polyolefin Zeonex 480 R
(average Mw ≈ 480,000, pellets) from Zeon Chemicals L.P.
2.2. Synthesis
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2.2.1. General Procedure for compounds 1 and 2
Sixty per cent NaH in oil was washed with hexane and then added to a stirred solution
of phenothiazine 5 or phenoxazine 6 derivative (10.0 mmol) in dry THF (15 mL) under a
nitrogen atmosphere at room temperature. After stirring for 30 min, 4-fluoro-2-(fluoromethyl)-
benzonitrile 7 was added.
The reaction mixture was quenched with water (30 mL). The organic phase was
separated, dried over anhydrous MgSO4, filtered and concentrated under reduced pressure.
The residue obtained was purified by column chromatography using a mixture of hexane/ethyl
acetate (3:1) as eluent.
2.2.1.1. 4-Phenothiazin-10-yl-2-trifluoromethyl-benzonitrile (1)
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The general method was used with 5 (0.23g, 1.16 mmol) in dry THF (15 mL) under
nitrogen atmosphere at room temperature. After stirring for 30 min, 4-fluoro-2-(fluoromethyl)-
benzonitrile 7 (0.20g, 1.05 mmol) was added to afford 1 (0.094 g, 24%) as a yellow powder.
m.p.: 163-165 °C. IR νmax (KBr): 2363, 2225(C≡N); 1613,1430 (C=C aromatic ring); 1340,1250
(C-F); 950, 880, 818 (aromatic ring C-H); 760, 735, 695 (CF3); 627, 580 (C-S) and 545, 450 (C-
N) cm-1.1H NMR
(2H, t, J = 8.0 Hz), 7.68 (4H, d, J = 8.0 Hz), 7.93 (1H, d, 8.0 Hz) ppm; 13C NMR
δ (400 MHz; DMSO): 7.22 (2H, d, J = 6.0 Hz), 7.40 (2H, t, J = 8.0 Hz), 7.53
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δ (400 MHz;
DMSO) 149.6, 139.7, 137.5, 134.1, 132.8, 132.4, 129.8, 128.7, 128.0, 127.8, 124.0, 121.5,
117.2, 116.4, 111.5, 111.4, 98.7 ppm; HRMS (ESI): found [M+H]+ 368.0594, molecular formula
C20H11N2F3S requires 368.0595.
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2.2.1.2. 4-Phenoxazin-10-yl-2-trifluoromethyl-benzonitrile (3)
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The general method was used with 6 (0.213g, 1.16 mmol) in dry THF (15 mL) under nitrogen
atmosphere at room temperature. After stirring for 30 min, 4-fluoro-2-(fluoromethyl)-benzonitrile
7 (0.20g, 1.14 mmol) was added to afford 3 (0.095 g, 26%) as a yellow powder. m.p.: 55-57 °C.
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IR νmax (KBr): 2232 (C≡N) 1273, 915(C-F) 670-610 (CF3). H NMR δ (400 MHz; DMSO): 7.59
(2H, d, J = 6.0 Hz), 7.25(2H, d, J = 9.0 Hz), 7.12 (2H, m, , J = 8.0 Hz), 7.02 (4H, m), 5.95 (2H,
+
d, J = 8.0 Hz) ppm. HRMS (ESI): found [M+H] 352.0834, molecular formula C20H11N2F3O
requires 352.823. 13C NMR δ (400 MHz; DMSO): 146.6, 145.7, 132.9, 130.5, 128.3, 125.4,
125.2, 119.3, 119.1, 117.4, 108.8 ppm.
2.2.2. General Procedure for Buchwald- Hartwig cross-coupling for compounds 2 and 4
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A mixture of phenothiazine 5 or phenoxazine 6 (1.0 eq.), 4-bromobenzonitrile 8 (1.2 eq.), K2CO3
(5.0 eq.), Pd(OAc)2 (0.2 eq.), P(t-Bu)3 (0.3 eq.) and dry toluene (20 mL) were stirred at 120 ºC in
nitrogen atmosphere for 24 hours. After cooling, water was added and the product was
extracted with ethyl acetate (2 × 30 mL). The organic layer was washed with brine solution and
dried over anhydrous MgSO4. After filtration, the solvent was removed under vacuum and the
residue was purified by column chromatography using a mixture of petroleum ether/CH2Cl2 (8:2)
as eluent.
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2.2.2.1. 4-Phenothiazin-10-yl-benzonitrile (2)
The general method was used with phenothiazine 5 (0.3 g, 1.5 mmol), 4-bromobenzonitrile 8
(0.32 g, 1.8 mmol), K2CO3 (1.02 g, 7.52 mmol), Pd(OAc)2 (0.047 g, 0.21 mmol) and P(t-Bu)3 (0.1
ml ,0.45 mmol) to afford 2 (0,115g, 25%) as a yellow solid. m.p.: 145-147 ºC. IR νmax (KBr):
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2212(C≡N) cm-1 .1H NMR δ (400 MHz; DMSO): 7.10 (2H, d, J = 6.0 Hz), 7.29 (4H, m, J = 9.0
Hz), 7.38 (2H, t, J = 8.0 Hz),7.53 (2H, d, J = 4.0 Hz),7.72 (2H, d, 8.0 Hz) ppm. 13C NMR δ (400
MHz; DMSO): 148.8, 141.0, 134.6, 134.3, 131.7, 129.2 ,129.0, 128.4, 128.3, 128.2, 126.7,
126.0, 125.8, 119.0, 118.0, 117.8, 104.7, ppm. HRMS (ESI): found [M+H] + 300.0725, molecular
formula C19H12N2S requires 300.0721.
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2.2.2.2. 4-Phenoxazin-10-yl-benzonitrile (4)
The general method was used with phenoxazine 6 (0.3 g, 1.6 mmol), 4-bromobenzonitrile 8
(0.36 g, 1.9 mmol), K2CO3 (1.13 g, 8.19 mmol), Pd(OAc)2 (0.074 g, 0.32 mmol) and P(t-Bu)3 (0.1
ml ,0.49 mmol) to afford 4 (0,110g, 25%) as a yellow-brown solid. m.p.: 65-67 ºC. IR νmax (KBr):
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2230(C≡N) cm-1 .1H NMR (400 MHz; DMSO): 8.14 (2H, d, J = 9.0 Hz), 7.68 (2H, d, J = 9.0 Hz),
6.74 (6H, m, J = 8Hz), 5.93 (2H, d, J = 4.0 Hz); 13C NMR δ (400 MHz; DMSO): 143.7, 143.3,
135.9, 133.5, 132.2, 129.9, 129.3, 128.7, 128.5, 124.3, 122.6, 118.8, 116.0, 114.0 ppm. HRMS
(ESI): found [M+H] + 284.0956, molecular formula C19H12N2O requires 284.0950.
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Instruments and measurement conditions
1H and 13C NMR spectra were recorded on a Bruker Avance III 400 operating at 400 MHz.
Chemical shifts are reported in ppm downfield from tetramethylsilane, and coupling constants
(J) are reported in Hz. Melting points were measured on a Reichert Thermovar apparatus fitted
with a microscope and are uncorrected. The absorption spectra were recorded at room
temperature in quartz cuvettes using a Jasco V-660 UV-Vis spectrophotometer and the
fluorescence measurements were obtained using a Horiba Jobin Yvon Fluorolog 3
spectrofluorimeter. Fluorescence quantum yields of compounds 1, 2, 3 and 4 were determined
by comparative method at 298 K using anthracene in ethanol (Φ = 0.21) as fluorescence
standard. The sample-containing Zeonex films were prepared by dissolving the compounds (ca.
1.0 mg) and the polymer (100 mg) in toluene (1.0 mL) and then by coating a quartz plate with
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the viscous solution at room temperature. After complete solvent evaporation, the plate
containing the clear film was placed in a quartz cell that was degassed with an Alcatel Drytel
100 turbomolecular pump (final pressure: ca. 3×10-7 atm), the cell being sealed afterwards. The
luminescence spectra and lifetime decays in Zeonex films were obtained with a Horiba Jobin
Yvon Fluorolog 3 spectrofluorimeter in right-angle configuration. Emission spectra were
corrected for the spectral response of the optics and photomultiplier. The films were mounted
slightly away from a 45º angle geometry to minimize specular reflection of the excitation light.
Time-resolved fluorescence intensity decays in solution were obtained by the single-photon
timing method with laser excitation, with the setup described in Ref. [21]. Measurements at 77 K
were performed with a liquid nitrogen cryostat (Oxford DN1704) connected to an ITC-4
temperature controller. Solutions of the samples were contained in 1 cm path-length quartz cells
at the optical density below 0.1 for fluorescence studies. Cyclic voltammetry (CV) was carried
out at room temperature using a three electrode cell in potentiostat/galvanostat Gamry
Instrument reference 600. The scan rate was 50 mV/s. A platinum disk, platinum wire and
Ag/AgCl (3 M KCl solution) were used as working (WE), counter (CE) and reference electrodes,
respectively. The supporting electrolyte was a solution of 0.1 M tetrabutylammonium
hexafluorophosphate (TBAPF6) in anhydrous N,N-dimethylformaldehyde, with concentration of
dyes fixed at 1 × 10-3 M. The potential was calibrated against ferrocene/ferrocenium couple
(Fc/Fc+). The HOMO energy level was calculated using the following equation: EHOMO= -
(EOXonset + 4.8) eV, where Eoxonset is the onset oxidation potential versus Ag/Ag+ [15].
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3. Results and Discussion
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3.1. Synthesis
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The synthetic route towards 1, 2, 3 and 4 is shown in Scheme 1 and the detailed synthetic
procedures are described in the Experimental Section.
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Scheme 1: Synthetic methodologies utilized to obtain derivatives 1 to 4.
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The synthesis of 1 and 3 was performed according to the synthetic method reported
previously for pyrrole and fluorobenzene derivatives. 1 and 3 were synthesized by reaction of
phenothiazine or phenoxazine with a fluorobenzene derivative at room temperature for 24h
under a nitrogen atmosphere. The synthesis of 2 and 4 were achieved by Buchwald-Hartwig
cross-coupling reactions between phenothiazine or phenoxazine and 4-bromobenzonitrile.
All the new compounds were characterized by IR, NMR, and high-resolution mass
spectral analysis. The spectral data were consistent with the proposed structures. The
compounds are reasonably soluble in dichloromethane, chloroform, tetrahydrofuran,
dimethylformamide and toluene.
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3.2. Optical, Photophysical and Electrochemical properties
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The absorption data (maximum absorption wavelengths and corresponding molar
absorption coefficients) are collected in Table 1. Fluorescence quantum yields and lifetimes in
aerated toluene solution were also determined, and the corresponding radiative and non-
radiative constants computed. No significant changes are observed upon degassing. All the
results are summarized in Table 1.
Table 1: Optical and photophysical properties
λmax (abs)/nm
(ε/M-1 cm-1)
λmax(em)/nm
ΦF/10-2
τ/ns
Compound
kr/s-1
knr/s-1
1
2
3
313 (11900)
300 (10100)
564
498
547
2.3
1.7
3.1
12.5
2.7
1.8×106
6.3×106
1.1×107
7.8×107
3.6×108
3.5×108
320 (14100)
401 (2100)
2.8
4
321 (6300)
372 (1600)
487
7.0
11.2
6.3×106
8.3×107
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Absorption and emission spectra of the four compounds are represented in Figures 1
and 2, respectively.
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Figure 1 – Normalized absorption spectra of compounds 1 (3.0 × 10-5 M), 2 (3.7 × 10-5 M), 3
(3.2 × 10-5 M) and 4 (3.9 × 10-5 M) in 1,4-dioxane.
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Figure 2 – Normalized emission spectra of compounds 1 (3.0 × 10-5 M, λexc = 313 nm), 2 (3.7 ×
10-5 M, λexc = 300 nm), 3 (3.2 × 10-5 M, λexc = 430 nm) and 4 (3.9 × 10-5 M, λexc = 384 nm) in
toluene.
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The electrochemical properties of 1, 2, 3 and 4 were investigated by cyclic voltammetry
(CV) in deoxygenated DMF solution containing 0.1M of TBAPF6 as supporting electrolyte, see
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Table 2. The redox potentials were measured against an Ag/Ag+ reference electrode and the
CV curves were calibrated using the ferrocene/ferrocenium (Fc/Fc+) redox couple as an external
standard which was measured under the same condition after the measurement of samples.
The half-wave potential of Fc/Fc+ in DMF was 0.507 V.
The HOMO energy level was calculated using the following equation: EHOMO= - (Eoxonset + 4.8)
eV, where Eoxonset is the onset oxidation potential versus Ag/Ag+ [15].
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Table 2: Electrochemical properties of compounds 1 to 4
Compounds
EHOMO / eVa
-6.29
ELUMO / eVb
-3.96
1
2
3
4
-6.20
-3.11
-6.46
-3.38
-6.27
-3.45
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a Deduced from the equation EHOMO = - (Eoxonset + 4.8).
b Deduced from the equation ELUMO = - (Eredonset + 4.8).
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After the complete characterization of the compounds in solution, drop-cast solid films in
Zeonex were prepared in order to investigate their TADF properties.
Delayed emission spectra of the degassed solid films were obtained at different
temperatures, and the corresponding delayed fluorescence decays were measured. The TADF
properties of the compounds were found to be reversible, when performing a full temperature
cycle. The calculation of the singlet-triplet energy gap, ∆EST, was performed using equation (1),
1
1
ꢀꢃꢄ
∆ꢇꢈꢉ
ꢊꢋꢌ
=
+ ꢅexp ꢆ−
ꢍ
(1)
ꢀꢁꢂ
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where ꢀꢁꢂ is the delayed fluorescence lifetime and ꢀꢃꢄ is the hypothetical phosphorescence
lifetime in the absence of TADF [22]. Rearranging this equation as
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1
ꢀꢃꢄ
∆ꢇꢈꢉ
ꢊꢋꢌ
ln ꢎ
−
ꢏ = ln ꢅ −
ꢀꢁꢂ
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and plotting ln (1/ꢀꢁꢂꢐ vs 1/T with ꢀꢃꢄ chosen to give the best straight line, it is possible to
determine the value of ∆EST for each of the compounds 1 to 4, see Table 3.
The delayed emission spectra of compounds 2 and 4 are represented in Figures 3 and
4 (only the spectra corresponding to temperatures where TADF behavior was observed are
represented).
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Figure 3 – Delayed emission spectra of 2 in Zeonex film between 138 and 303 K. Inset:
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variation of the intensity at 494 nm.
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Figure 4 – Delayed emission spectra of 4 in Zeonex film between 77 and 198 K. Inset: variation
of the intensity at 476 nm.
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The delayed fluorescence lifetimes and corresponding plots of ln (1/ꢀꢁꢂꢐ vs 1/T are
represented in Figures 5 and 6, and respective insets.
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Figure 5 – Delayed fluorescence lifetimes of 2 in Zeonex film at different temperatures. Inset: ln
(1/ꢀꢁꢂꢐ vs 1/T. ꢀꢃꢄ = 104 ms.
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Figure 6 – Delayed fluorescence lifetimes of 4 in Zeonex film at different temperatures. Inset: ln
(1/ꢀꢁꢂꢐ vs 1/T. ꢀꢃꢄ = 190 ms.
The obtained values for ∆EST are summarized in Table 3.
Table 3: Calculated ∆EST values for compounds 1 to 4
Compound
∆EST / kJ mol-1 ∆EST / meV
1
2
3
4
2.9
8.2
30
85
5.6
58
10.1
105
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Considering the values of Tables 1 and 3, and the molecular structures represented on
Scheme 1, two approaches to the discussion can be followed. By comparing molecules bearing
the same donor group, but different substituents in the acceptor part, it is possible to observe
that the ones containing a trifluoromethyl substituent exhibit lower ∆EST values than their
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counterparts (1 compared to 2, and 3 compared to 4). On the other hand, comparing derivatives
with the same acceptor group but different heteroatoms in the donor moiety, we find that
phenothiazines display lower ∆EST values than their corresponding phenoxazine analogues. In
both cases, the electron-withdrawing forces play a key role, for the singlet-triplet energy splitting
is reduced when strong acceptors are present in the molecule (trifluoromethyl is a very strong
electron-withdrawing group), but also good donors contribute to this reduction (and oxygen,
being more electronegative than sulfur, is a poorer electron donor).
Globally, all four DA compounds display low ∆EST values, which makes them
candidates for OLED applications. Further photophysical studies in this regard are nevertheless
required.
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Conclusion
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Four new phenoxazine-benzonitrile and phenothiazine-benzonitrile luminescent DA
molecules were synthesized and fully characterized. All display TADF at temperatures well
below room temperature owing to small singlet-triplet (∆EST) energy gaps and long intrinsic
triplet lifetimes.
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Acknowledgements
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Support for this work was provided by Fundação para a Ciência e a Tecnologia (FCT, Portugal)
through project FAPESP/20107/2014. B. Pedras acknowledges FCT-Portugal and IST-ID for
the research contract IST-ID/104/2018. E. Torres acknowledges funding from I&D Point4Pac
(Lisboa-01-0145-FEDER-016405).
.
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Activated Delayed Fluorescence, H. Yersin ed. pp. 225-252, Wiley-VCH, 2018.
Captions
Scheme 1: Synthetic methodologies utilized to obtain derivatives 1 to 4.
Table 1: Optical and photophysical properties.
Figure 1 – Normalized absorption spectra of compounds 1 (3.0 × 10-5 M), 2 (3.7 × 10-5 M), 3
(3.2 × 10-5 M) and 4 (3.9 × 10-5 M) in 1,4-dioxane.
Figure 2 – Normalized emission spectra of compounds 1 (3.0 × 10-5 M, λexc = 313 nm), 2 (3.7 ×
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10-5 M, λexc = 300 nm), 3 (3.2 × 10-5 M, λexc = 430 nm) and 4 (3.9 × 10-5 M, λexc = 384 nm) in
toluene.
Table 2: Electrochemical properties of compounds 1 to 4
Figure 3 – Delayed emission spectra of 2 in Zeonex film between 138 and 303 K. Inset:
variation of the intensity at 494 nm.
Figure 4 – Delayed emission spectra of 4 in Zeonex film between 77 and 198 K. Inset: variation
of the intensity at 476 nm.
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Figure 5 – Delayed fluorescence lifetimes of 2 in Zeonex film at different temperatures. Inset: ln
(1/ꢀꢁꢂꢐ vs 1/T. ꢀꢃꢄ = 104 ms.
Figure 6 – Delayed fluorescence lifetimes of 4 in Zeonex film at different temperatures. Inset: ln
(1/ꢀꢁꢂꢐ vs 1/T. ꢀꢃꢄ = 190 ms.
Table 3: Calculated ∆EST values for compounds 1 to 4
Figure SM1 - Cyclic voltammograms of compounds 1 to 4 in DMF solutions with TBAPF6 (0.1
M) as electrolyte; scan rate: 50 mV s-1. All voltammograms were referenced to internal
ferrocene (Fc/Fc+)
Figure SM2 - 1H NMR spectrum of 4-Phenothiazin-10-yl-2-trifluoromethyl-benzonitrile (1)
Figure SM3 - 13C NMR spectrum of 4-Phenothiazin-10-yl-2-trifluoromethyl-benzonitrile (1)
Figure SM4 - 1H NMR spectrum of 4-Phenothiazin-10-yl-benzonitrile (2)
Figure SM5 - 13C NMR spectrum of 4-Phenothiazin-10-yl-benzonitrile (2)
Figure SM6 - 1H NMR spectrum of 4- Phenoxazin-10-yl-2-trifluoromethyl-benzonitrile (3)
Figure SM7 - 13C NMR spectrum of 4-Phenoxazin-10-yl-2-trifluoromethyl-benzonitrile (3)
Figure SM8 - 1H NMR spectrum of 4-Phenoxazin-10-yl-benzonitrile (4)
Figure SM9 - 13C NMR spectrum of 4-Phenoxazin-10-yl-benzonitrile (4)
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Supplementary material
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Figure SM1 - Cyclic voltammograms of compounds 1 to 4 in DMF solutions with TBAPF6 (0.1
M) as electrolyte; scan rate: 50 mV s-1. All voltammograms were referenced to internal
ferrocene (Fc/Fc+)
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Figure SM2 - 1H NMR spectrum of 4-Phenothiazin-10-yl-2-trifluoromethyl-benzonitrile (1)
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Figure SM3 - 13C NMR spectrum of 4-Phenothiazin-10-yl-2-trifluoromethyl-benzonitrile (1)
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Figure SM4 - 1H NMR spectrum of 4-Phenothiazin-10-yl-benzonitrile (2)
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Figure SM5 - 13C NMR spectrum of 4-Phenothiazin-10-yl-benzonitrile (2)
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Figure SM6 - 1H NMR spectrum of 4- Phenoxazin-10-yl-2-trifluoromethyl-benzonitrile (3)
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Figure SM7 - 13C NMR spectrum of 4-Phenoxazin-10-yl-2-trifluoromethyl-benzonitrile (3)
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Figure SM8 - 1H NMR spectrum of 4-Phenoxazin-10-yl-benzonitrile (4)
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Figure SM9 - 13C NMR spectrum of 4-Phenoxazin-10-yl-benzonitrile (4)
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Highlights
•
Four new donor-acceptor (DA) molecules of the phenothiazine-benzonitrile and
phenoxazine-benzonitrile types.
•
•
All DA molecules display thermally activated delayed fluorescence
All DA molecules have a small singlet-triplet energy gap

Author statement
All the authors contributed equally to the work described in this manuscript.

Declaration of interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be considered
as potential competing interests:
None.